1. Field of the Invention
The present invention relates to a parallel axes type automatic transmission having a plurality of shafts extending in parallel to each other and a plurality of gears provided on the shafts and meshing with each other, wherein the connection and disconnection between each shaft and the gears provided thereon are made to thereby switch between power transmitting routes formed between the shafts, thereby obtaining a desired gear ratio.
2. Description of the Related Art
Such a parallel axes type transmission is used not only in a vehicle, but also in various power machines. In such a transmission for a vehicle, the number of shift steps tends to be increased to meet the requirements for a vehicle driving performance and effects on the environment, and a transmission having five or more forward speeds has already been put into practical use. Generally in a transmission not limited to a transmission for a vehicle, the number of gears provided on each shaft increases with an increase in number of shift steps, so that the overall size of the transmission, particularly the axial size thereof tends to increase. A transmission is required to be installed in a certain space defined in a machine using this transmission. Accordingly, in the case that the number of shift steps increases to cause an increase in axial size of the transmission, it is necessary to make the transmission compact by adopting any means. Particularly in a vehicle transmission limited in various sizes from the viewpoint of the size of a vehicle body, making the transmission compact is a very important issue.
Various inventions have conventionally made to reduce the axial size of a parallel axes type transmission. For example, an intermediate shaft is interposed between an input shaft and an output shaft, and the intermediate shaft is driven through gears by the input shaft, whereby the number of gears provided on each shaft is reduced to thereby reduce the axial size of the transmission (see Japanese Patent Laid-open No. 2000-220700, for example). Further, in addition to the above configuration, it is known that the gear provided on the output shaft and meshing with the gear provided on the input shaft is the same as the gear provided on the output shaft and meshing with the gear provided on the intermediate shaft (see Japanese Patent Publication No. Hei 7-94854, for example).
In this configuration, the gears provided on the output shaft for providing two different gear ratios are provided by a common gear. As a result, the number of gears provided on the output shaft can be reduced, and the gears provided on the input shaft and the intermediate shaft and meshing with the common gear provided on the output shaft can be arranged in substantially the same plane. As a result, the axial size of the transmission can be greatly reduced. Further, as disclosed in Japanese Patent Laid-open No. 2005-54958, it is known in the art that an idle gear train is used to drive the intermediate shaft and that the number of shift steps can be increased and the axial size can be reduced by increasing the number of intermediate shafts with the use of idle gear trains.
However, in the configuration described in Japanese Patent Publication No. Hei 7-94854 wherein the common gear for two different gear ratios is provided on the output shaft, there is a case that the effect of reducing the axial size is insufficient in a recent transmission having a tendency to increase the number of shift steps, and it is therefore desired to provide a configuration which can reduce the axial size more greatly. Further, the parallel axes type transmission described in Japanese Patent Laid-open No. 2005-54958 has a compact structure with a reduced axial size for the number of shift steps. However, it is necessary to provide a plurality of wet multiplate clutches whose number is equal to the number of shift steps, causing a somewhat complicated configuration.